In recent years, the decrease in impedance (equivalent series resistance) of a capacitor to be used in electronic equipment is required in a high frequency area in accordance with the digitalization of electronic equipment. In order to fulfill the requirement, a functional capacitor (hereinafter, abbreviated as “capacitor”) having a coating film formed by oxidizing valve metal, such as aluminium, tantalum, or niobium, as a dielectric substance, has been conventionally used.
Such a capacitor generally has a structure composed of an anode made of porous valve metal, a dielectric layer formed by oxidizing the surface of the anode, a conductive solid electrolyte layer, and a cathode formed by laminating a carbon-layer, a silver-layer, and the like, as shown in Patent Document 1. There is a case in which a conductive film containing a π conjugated conductive polymer is used as the solid electrolyte layer.
An electrolytic polymerization method in which conductive layers of manganese oxide previously formed on the surface of porous valve metal are used as electrodes and an electric current is applied thereto for polymerization (see Patent Document 2), and a chemical oxidation polymerization method in which precursor monomers which serve for a π conjugated conductive polymer are polymerized using an oxidant (see, Patent Document 3) have been widely known as the method for forming the conductive film containing a π conjugated conductive polymer.
In addition to the electrolytic polymerization method and the chemical oxidation polymerization method, a method in which aniline is subjected to chemical oxidation polymerization in the presence of polyanion having a sulfonic group, a carboxylic group, or the like, and the thus obtained solution of water-soluble polyaniline is applied and dried to form a coating film, has been proposed in Patent Document 4, for example, as the method for forming a conductive film. It is considered that a conductive film with high conductivity can be easily formed by such a method.
Patent Document 1: Japanese Laid-Open Patent Application No. 2003-37024.
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. S63-158829.
Patent Document 3: Japanese Unexamined Patent Application, First Publication No. S63-173313.
Patent Document 4: Japanese Unexamined Patent Application, First Publication No. H7-105718.